For messages and pic, not talk, WhatsApp uses the same internet data plan as web browsing and email on your phone. If using your cellular data while roaming will normally result in additional charges, you may be charged extra by your mobile provider for using WhatsApp while roaming.
However, provided you have Wi-Fi internet access and current WhatsApp service, WhatsApp should be free to use over a Wi-Fi connection. Make sure your data roaming is disabled, though.